"This incident was not the fault of a single individual or organization, but rather a systemic failure across organizations and agencies," the president said at the White House. "Ultimately, the buck stops with me."
Obama made his remarks as his administration released a declassified version of its review of the Christmas Day plane bomb plot, which detailed the "human and systemic" failures that allegedly allowed Umar Farouk Abdulmutallab to board a U.S.-bound plane with explosives and pointed a finger at two key intelligence agencies.
While the entire intelligence community has received some criticism from the administration in recent days, the report focused considerable scrutiny upon the National Counterterrorism Center and the CIA.
"Though all the information was available to all source analysts at the CIA and NCTC prior to the attempted attack, the dots were never connected," the report reads. "As a result, the problem appears to be more about a component failure to connect the dots, rather than a lack of information sharing."
The counterterrorism center, which was designed to put the nation's 16 intelligence and law enforcement agencies in one building and force them to work together, failed to take aggressive action after receiving a State Department cable in November that a prominent Nigerian businessman warned about his son being radicalized by extremists in Yemen, according to the report.
Intelligence gathered by the CIA on the Nigerian bombing suspect aboard Northwest flight 253 also was not disseminated in an urgent manner to the counterterrorism center, the review concluded.
In his remarks, Obama once again reiterated a key finding of the report and a point he made earlier this week -- that the government failed to connect the dots.
"Rather than a failure to collect or share intelligence, this was a failure to connect and understand the intelligence that we already had," Obama said.
Obama's top counterterrorism adviser, John Brennan, was candid in his analysis of the system's failures.
"I told the president that I let him down," Brennan said in a briefing at the White House. "I told him that I will do better and we will do better as a team."
Over the past year, Brennan added, the intelligence community has "done a stellar job in protecting the homeland," but, "in this one instance we did not rise to that same level of competence."
The president outlined additional steps he has ordered across multiple government agencies to correct mistakes made in the lead-up to the Christmas Day plot.
The directives included quicker and wider distribution of intelligence reports, strengthening the process by which intelligence analysts process and integrate information and strengthening the criteria used to add to terrorist watch lists.
Homeland Security Secretary Janet Napolitano announced that later this month she'll be traveling to Spain to meet with her European counterparts for what will be the first in a series of meetings seeking "a broad consensus on new international aviation security standards and procedures."
Napolitano said DHS also will accelerate deployment of advanced imaging technologies, "so that we have greater capabilities to detect explosives like the ones used in the Christmas Day attack."
"We must do better in keeping dangerous people off airplanes, while still facilitating air travel," Obama said.
But the president acknowledged that there is no "foolproof solution" to prevent future attacks.
"As we develop new screening technologies and procedures, our adversaries will seek new ways to evade them, as was shown by the Christmas attack," he said. "In the never-ending race to protect our country, we have to stay one step ahead of a nimble adversary."
Obama's news conference had been twice delayed Thursday as intelligence officials asked for more time to "scrub" the review to make sure the unclassified version was acceptable for public release, according to sources.
Obama was first scheduled to speak at 1 p.m., then 3 p.m. and finally spoke just after 4:30 p.m. ET at the White House.
Several intelligence officials reached by ABC News conceded the missed clues but said the information received in the field was not believed to be specific or urgent enough to merit high priority at the time.
In a statement, CIA spokesman George Little defended his agency in the face of the report's conclusions, saying that before the Detroit incident, "the CIA collected and shared information about Abdulmutallab with other agencies."
On Jan. 5, CIA Director Leon Panetta instructed the agency to implement a series of reforms immediately, including new procedures to disseminate intelligence on terror suspects, expansion of name traces, and greater focus on Yemen and Africa.
The White House had indicated today's release of the so-called after-action report on the Dec. 25 incident would close the door on the investigation but that the overall review of security and intelligence procedures and structures would continue.
White House press secretary Robert Gibbs said Wednesday that the president will want to "continue to look at whether or not the progress on what has been identified and what will change [takes place and] whether we're making progress in meeting those necessary changes."
Republicans have blasted the administration for its handling of the incident.
Former vice president Dick Cheney has said Obama is "trying to pretend" the United States is not "at war."
"I think in the last three days he has [done enough]," Republican strategist Nicole Wallace said of Obama on "Good Morning America."
"[But] in the early hours after the failed terror attack, he took about six more rounds of golf and a movie with the kids," she added. "I think that first impression was that he was slow, that his surrogates were out there saying the system worked, and I think those things do irreparable harm."
Former 9/11 commission chairman Lee Hamilton told ABC News Wednesday that while the president does bear the burden of responsibility for preventing such attacks, the problem of lax vigilance throughout the government is primarily to blame.
"I just think what's pervasive through the country, and has been now for a number of years, is the complacency, an inertia, a business-as-usual attitude ... that I think is harmful," Hamilton said.
That complacent attitude, he said, includes the entire political leadership of the United States -- Obama, congressional leaders and the "many, many people that have had a part in homeland security."
In the wake of the attempted Dec. 25 attack, Congress is expected to hold a number of hearings on homeland security when it returns from its winter recess later this month.
Republican Sen. Susan Collins, R-Maine, the ranking member of the Senate Homeland Security Committee, was critical of Obama's remarks Thursday, saying in a statement that the president's response "falls short" when it comes to stopping potential terrorists from entering the country.
Collins called on the State Department to suspend visas of everyone in the "broadest terrorist database," pending further investigation. Her committee is slated to consider the Christmas bomb plot in a hearing on Jan. 21.
House Homeland Security Committee chairman Rep. Bennie Thompson, D-Miss., announced today that his panel would hold a hearing on the plot on Jan. 27.
"The hearing will give us a unique opportunity to provide the public with insight on this event and the security failures that led to what the president has appropriately labeled a systemic failure," he said.
Sen. Patrick Leahy, D-Vt., who chairs the Senate Judiciary Committee, called the findings of the administration's security review a "chilling narrative" reminiscent of the failures leading to Sept. 11.
"Eight years later, inadequate analysis of critical information remains of paramount concern," he said, also promising to hold oversight hearings this month.
The Senate Intelligence Committee, scheduled to discuss worldwide threats to the U.S. on Feb. 2, is also likely to address the incident and intelligence shortfalls.
Obama's strongest previous comments about the Christmas Day incident came Tuesday when he said that the nation's security and intelligence systems "failed in a potentially disastrous way" when Abdulmutallab allegedly was able to board a U.S.-bound flight with explosives strapped to his underwear.
He told members of his national security team behind closed doors that the intelligence failure around the attempted Christmas day attack was a "screw-up" and that they only dodged a bullet because of brave individuals of Flight 253.
"This was a screw-up that could have been disastrous," the president said in the Situation Room, according to a White House official. "We dodged a bullet, but just barely. It was averted by brave individuals, not because the system worked, and that is not acceptable. While there will be a tendency for finger pointing, I will not tolerate it."
The president announced Tuesday "concrete steps" his administration has taken to enhance security, including "more air marshals on flights."
In Detroit Wednesday, Abdulmutallab was indicted on charges that included attempted murder and trying to use a weapon of mass destruction to kill nearly 300 people.
Abdulmutallab, who faces life in prison if convicted, is expected to appear in federal court Friday.